1. Field of the Disclosure
The present disclosure relates to electronic devices and methods and, more particularly, to electronic devices including nonvolatile memory arrays and methods of using the same.
2. Description of the Related Art
Nonvolatile memory (“NVM”) arrays are used in electronic devices. Erasing of NVM arrays is performed by causing charges to tunnel through a dielectric layer (“tunnel oxide”) of a charge-storage region (e.g. a floating gate, discontinuous storage elements, etc.). Tunneling charges can cause charge-trapping sites to form within the tunnel oxide. Charge-trapping sites build up and can eventually degrade the insulating properties of the tunnel oxide to the point that the charge-storage region can no longer store charge. Thus, by reducing the cumulative number of erase pulses performed on an NVM array, the life of the NVM array can be extend.
An NVM array can be designed so that erasing can be done on a memory cell by memory cell basis. However, erasing is a much slower process than programming or reading the NVM array. Thus, for most applications, overall performance and area can be improved by erasing the NVM array in blocks (“sectors”) rather than one memory cell at a time. In one example, performing the erasing sequence on a sector of the NVM array includes applying a voltage of greater than 12 V to the control gate line of the individual memory cells of a sector relative to a bulk voltage of 0 V. During an erasing sequence, all memory cells of a sector of the NVM array are erased simultaneously. Memory cells of the sector are then checked individually to verify the erase of each memory cell was completed. If any memory cell of the sector is not completely erased, all the memory cells of the sector are further erased, including any memory cell previously verified as erased. The erasing sequence ends when all memory cells are verified as having completed erasing. In such a case, all memory cells in the sector receive an equal number of erase pulses.
However, the number of erasing pulses to completely erase a memory cell can vary within the sector. Thus, within a sector there can be a first-to-erase memory cell, which is the memory cell that is actually the first to become erased, and a last-to-erase memory cell, which is the memory cell that is actually the last to become erased. In the example, a last-to-erase memory cell may require more pulses to erase than a first-to-erase memory cell. Thus, the first-to-erase memory cell can be subjected to additional erasing pulses after verifying as having completed erasing. The additional erasing pulses shorten the potential life of the first-to-erase memory cell.
After the erasing the sector, the memory cells of the sector have a statistical distribution of threshold voltage (“Vth”) values. As the number of cells in the sector increases, the delta Vth between the first-to-erase and last-to-erase memory cells can increase reducing the write/erase endurance capability of the device. The size of the delta Vth is in large part controlled by the number of memory cells erased at one time. Therefore, control of the delta Vth can limit the size of a sector, and the erase performance of the NVM array.
Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the invention. The use of the same reference symbols in different drawings indicates similar or identical items.